WASHINGTON (AP) — An influential federal advisory panel has soundly rejected a plan to offer Pfizer booster shots against COVID-19 to most Americans.

The vote Friday, 16-2, was a blow to the Biden administration’s effort to shore up people’s protection against the virus amid the highly contagious delta variant.

Over several hours of discussion, members of the Food and Drug Administration panel of outside experts voiced frustration that Pfizer had provided little data on the safety of extra doses.

And they complained that data provided by Israeli researchers about their booster campaign might not be suitable for predicting the U.S. experience.

Scientists inside and outside the government have been divided in recent days over the need for boosters and who should get them, and the World Health Organization has strongly objected to rich nations giving a third round of shots when poor countries don’t have enough vaccines for their first.

The panel, made up of outside experts who advise the Food and Drug Administration, weighed a less than clear-cut case: While research suggests immunity levels in those who have been vaccinated wane over time and boosters can reverse that, the Pfizer vaccine is still highly protective against severe illness and death, even amid the spread of the highly contagious delta variant.